A previous study for the County Council had identified some traditional mitigation options, including creating engineered holding ponds in unidentified locations upstream of the village. Viridian were commissioned to identify the best Natural Flood Management (NFM) measures in the catchment to reduce flood impact to the village, as well as look at secondary benefits around water flows. A scheduled ancient monument close to the village had to be protected from any NFM measures.

Overview of the Fillongley catchment

The Bourne Brook and Didgeley Brook form a ‘flashy’ catchment, where heavy rainfall leads to rapid run-off and floods the village of Fillongley. The landscape is dominated by pasture and arable land, with woodland to the south and west of the village. The Fillongley catchment drains from south to north.

Viridian took a few days to research what had already been done in the area and model accordingly. One aspect of interest to the client was to identify the best locations for pond creation in the valley, with a view to minimising their engineering requirements and so cost of creation.

The outputs were maps showing the best new habitats to create and the best places to create them for least investment. This was done for flooding alone and then for a combined solution, where the combined solution balanced flood mitigation with planting for erosion control, groundwater recharge, and pollutant retention (both water soluble and soil adsorbed). The maps here showing exactly what should be done where, down to the nearest 5 metres, highlighting areas that deliver the most benefit.

Flood solution for Fillongley catchment

This map shows the optimal areas for natural flood mitigation for the catchment, plotted with deeper coloured shades corresponding to higher priority areas. One can see that in this area, wetland is the highly preferred intervention, either creating leaky dams, or forming ponds/swales, depending on local circumstances. Wetland is often selected for flood mitigation as it is effective at slowing flows across the landscape, reducing flow volumes downstream whilst offering a lower risk of synchronising peak flood heights than would planting trees.

Overall solution for Fillongley catchment

This map shows the optimal areas to plant habitat for a basket of ecosystem services (see above). The map has been plotted with different colours representing the different habitat choices and with deeper shades of colour corresponding to higher priority areas. One can see that existing green infrastructure, such as the woodlands to the south and west, lowers the priority for land use change in the immediate area (i.e., there are far fewer highlighted areas between the existing woodland and stream) as they already provide a good degree of ecosystem service to the current landscape.